1. Field of the Invention
The present invention relates to a device for measuring a nonreciprocal phase shift in a closed-loop optical interferometer which is also known as a Sagnac interferometer.
2. Description of the Prior Art
An interferometer of this type mainly comprises a source of light energy usually provided by a laser, an optical device forming a waveguide and constituted either by a predetermined number of mirrors or by a multiturn optical fiber coil, a light-beam splitting and mixing device, and a device for detecting and processing the detected signal.
In interferometers of this type, it is known that there are two waves which emerge from the beam-splitter and travel along the same optical path in opposite directions.
A basic property of closed-loop interferometers is reciprocity. This property can be expressed as follows: any disturbance of the optical path affects the two waves in a similar manner in spite of the fact that these two waves are subjected to the disturbance neither exactly at the same instant nor in the same direction.
There are, however, two types of disturbances which affect reciprocity.
One type consists of disturbances which vary with time, namely over a period comparable to the time taken by the waves to propagate along the optical path of the interferometer. The other type consists of the so-called "nonreciprocal" disturbances or, in other words, disturbances which produce a different effect on the waves according as they propagate in one direction or in another along the optical path. These are in fact physical effects which destroy the symmetry of the medium in which the waves propagate.
Two known effects are subject to this second type of disturbance:
the Faraday effect, or colinear magnetooptical effect, in which a magnetic field produces a preferential orientation of electron spin of an optical material; PA1 the Sagnac effect, or relativistic inertial effect, in which rotation of the interferometer with respect to a Galilean reference frame destroys the symmetry of the propagation time. This effect is advantageously employed in particular in the design and construction of gyrometers, usually known as rate gyros.
When "nonreciprocal" disturbances are not present, there is accordingly a zero phase difference (said difference being hereinafter designated as .DELTA..phi.) between the two waves which recombine in the light-beam splitting and mixing device after having traveled along the optical path. The detecting and processing device detects signals representing the optical power of the composite wave obtained after recombination. In interferometers of the prior art, the optical power just mentioned can be resolved into two components, namely a constant component and a component which is proportional to cos (.DELTA..phi.) and exists only at the time of appearance of "nonreciprocal" disturbances.
Should it be desired to measure low-amplitude disturbances such as, for example, low rotational or spin velocities in the case of rate gyros, the component which contains the term of the form cos (.DELTA..phi.) varies only to a slight extent since the phase shift .DELTA..phi. is close to zero.
It is accordingly necessary to introduce artificially a fixed additional phase shift or "non-reciprocal bias" for the purpose of increasing the sensitivity of measurement. In a particularly advantageous case, the new measured phase shift .DELTA..phi.' is such that .DELTA..phi.'=.DELTA..phi.+.pi./2.
In this case, maximum sensitivity is achieved since the term to be measured is proportional to cos (.DELTA..phi.+.pi./2), that is, to sin (.DELTA..phi.).
Although this method is attractive, practical difficulties have been encountered, especially in regard to the possibility of providing a device for introducing a nonreciprocal bias which is sufficiently stable to be suitable for use. The instability of these devices is usually of the same order of magnitude as the variations in the quantity to be measured.
In order to overcome these disadvantages, French patent No. FR-B-2,471,583 has consequently proposed a phase modulation of the waves which propagate within the closed loop, alternately of +.pi./2 and -.pi./2 radians.
This method is based on the property of the Sagnac interferometer which consists in producing the equivalent of a discrete temporal drift.
In fact, since a phase modulation is produced at one end of the fiber loop, one of the waves undergoes modulation at the moment of generation of the wave whereas the other wave experiences modulation with a time-delay equal to the time of propagation within the fiber. This propagation time satisfies the relation ##EQU1## where n is the refractive index of silica, 1 is the length of the fiber and C is the velocity of light in a vacuum. The "natural frequency" of the interferometer is (1/2 t.sub.o) and represents the modulation frequency at which the two waves undergo two phase shifts in phase opposition. The phase shift between the two optical waves is therefore equal to the difference S(t)-S(t-t.sub.o), where S(t) is the signal applied to the phase modulator. It is therefore apparent that, if the half-period of the modulating signal is t.sub.o, the phase shift at the exit of the interferometer is equal to twice the value of the applied phase shift. This is the method employed for producing the bias which serves to establish the operating point of the interferometer.
To this phase shift is added a phase shift .DELTA..phi..sub.o which arises from the nonreciprocal effect, namely from rotation if this latter is not zero.
It is possible to utilize the signals directly and to measure the component in cos (.DELTA..phi.+.pi./2).
A more accurate method which guards against errors arising from possible drift of the different elements employed such as the optoelectronic elements, for example, consists of an indirect or "zero method". In accordance with this method, the above-mentioned phase-shift difference with respect to .+-..pi./2 radians is compared by generating an additional phase shift which is equal at absolute value to the amplitude of the phase shift produced by the nonreciprocal effect of contrary sign in order to reduce it to zero.
In order to achieve this result, it is not possible in actual practice to utilize the same physical phenomenon which produces the nonreciprocal effect or in other words to modify the rotation.
Recourse is had to electrical means for generating a negative-feedback signal. This choice is based on the assumption that these electrical means can be controlled more effectively than the other elements of the interferometer, as has in fact been shown by practical experience.
The object of this negative feedback is to produce between the two waves a phase shift which is continuously equal and of opposite sign with respect to the phase shift induced by the rotational velocity. If the velocity is constant and produces a phase shift .DELTA..phi., it is therefore necessary to ensure that, between two separate instants of t, the instantaneous value of the phase modulation has varied by (.DELTA..phi..sub.o +2.pi.n) radians, where n is a whole number. This is therefore the equivalent of an integral of the velocity. One method of operation consists in generating a so-called "phase ramp" having a slope which is proportional to (.DELTA..phi..sub.o /t.sub.o).
However, this method presupposes two distinct operations: phase modulation and generation of a negative-feedback signal. Furthermore, the proportionality factor or scale factor is unrelated to that employed for the modulation of .+-.(.pi./2) radians.
Furthermore, the aforementioned phase ramp cannot be infinite. In other words, the signal which is constituted in practice by a phase modulator control voltage cannot increase above a predetermined threshold value.
In consequence, a feasible method consists in generating sawtooth phase-shift control signals having a peak-to-peak amplitude of 2.pi. radians, the mathematical functions involved being periodic and having a period of 2.pi. radians. There then follows the problem of accurately determining said phase-shift amplitude equal to 2.pi. radians.
The objective set by the present invention is to overcome the difficulties which have been mentioned in the foregoing. To this end, the so-called phase ramp consists of a digital signal. The phase modulation, also in digital form, and said phase ramp are combined in a single signal and converted to an analog signal for controlling a phase modulator placed within the interferometer loop.
Apart from the simplification which is achieved as a result of this arrangement, the relationship between scale factors and the problem of relaxation of 2.pi. radians are thus solved both simply and simultaneously.